Hybrid power output system

ABSTRACT

A hybrid power output system fore outputting the power to the wheel driving shaft ( 8 ), comprising an engine ( 1 ), a first motor ( 2 ), a second motor ( 3 ), a battery ( 6 ), a first clutch ( 4 ), a second clutch ( 5 ) and a third clutch ( 11 ), wherein: the first motor ( 2 ) and second motor ( 3 ) are connected electrically with the battery ( 6 ); the engine ( 1 ) is connected to the first motor ( 2 ) via the first clutch ( 4 ); the first motor ( 2 ) is connected to a wheel driving shaft ( 8 ) via the second clutch ( 4 ); the second motor ( 3 ) is connected to the wheel driving shaft via the third clutch ( 11 ); the second clutch and the third clutch are arranged between the first motor and second motor. This hybrid power output system is compact in structure, can increase the power efficiency and reduce the fuel consumption, and can realize multiple drive modes.

TECHNICAL FIELD

The present invention relates to a hybrid power output system, and more particularly to a hybrid power system for outputting the power to the wheel driving shaft to propel the vehicle.

BACKGROUND

The hybrid vehicle generally has two different power sources simultaneously. According to the various connection types, it may be mainly divided into three structures of series hybrid, parallel hybrid and series-parallel hybrid.

The series hybrid is characterized in electrically coupled, wherein the engine directly drives an electric generator for charging the battery, and then an electric motor is operated by electrical energy from the battery to drive the wheels. The parallel hybrid is characterized in mechanically coupled, wherein the engine is connected with the driving shaft, and the motor can be selectively operated as an electric motor or a electric generator for balancing the loads of the engine.

In the prior art, one of the series-parallel hybrid may use a planetary gear unit to realize the distribution of power among the driving units. In this structure, the engine may be connected to the carrier of the planetary gear unit, a part of the power of the engine is transferred into the electrical energy by means of the first motor for furnishing the electrical energy to the second motor or for charging the battery, and the other part of the power of the engine is directly transmitted to the ring gear of the planetary gear unit; meanwhile, the second motor is connected with the ring gear for the supply of some supplementary power or torque. In this structure, a part of the power output from the engine is always used to power the first motor to generate the electrical energy for charging the battery or for driving the second motor.

In the prior art, a series-parallel hybrid may use the clutch to realize the power distribution among the driving units. For example, the Chinese patent CN 1200824 C discloses a hybrid power drive system for propelling the vehicle, which comprises two clutches, two motors and a battery, wherein the first clutch and the second clutch are respectively disposed between the first motor, the second motor and the internal combustion engine, the second motor is permanently connected to a variable-ratio transmission, and the first motor and the second motor are connected electrically with the battery. In this patent, because the second motor is permanently connected to the variable-ratio transmission, that is, the second motor is permanently connected with the wheels via the transmission, it has the disadvantage in that: when the second motor fails to work due to some reasons (e.g. the electrical power is low, the second motor is breakdown or is controlled to stop) while the engine is started to drive the wheels, the engine will also drive the rotor of the second motor to rotate because the second motor cannot be disconnected with the wheel driving shaft. Thus, the second motor becomes to be a load to the driving system and, as a result, the energy is wasted and the driving efficiency is lowered.

Moreover, as it can be seen from the disclosure, the first motor of the drive system has three functions as below: (1) starting motor for the internal combustion engine; (2) generator for outputting electrical energy; and (3) drive device for driving the transmission hydraulic pump and other auxiliaries such as an air conditioner compressor, a power steering pump. Thus it can be seen that, the first motor is equal to a starting motor (with lower power or capacity) for the internal combustion engine in a conventional vehicle, except that it is electrically connected with the battery, and under control, it has some other functions besides to start the internal combustion engine, for example, to drive some auxiliaries, but it cannot be operated to drive the wheels alone or together with the second motor. Therefore, the drive system has limited driving modes and functions, and the power efficiency cannot be optimized. In addition, although the second motor can be operated to drive the wheels in a purely electrical manner, it can not work always in a high efficiency when it is used to drive the wheels in various conditions. If it intends to improve the efficiency, the demand for the design of the motor is high and difficult to meet.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a hybrid power output system, which can effectively reduce the power waste, enhance the power performance, realize multiple driving modes, increase the efficiency and reduce the fuel consumption.

In order to achieve the object mentioned above, according to the present invention, a hybrid power output system is provided, which comprises an engine, a first motor, a second motor, a battery, a first clutch, a second clutch and a third clutch, wherein: the first motor and the second motor are connected electrically with the battery; the engine is connected to the first motor via the first clutch; the first motor is connected to a wheel driving shaft via the second clutch; the second motor is connected to the wheel driving shaft via the third clutch; the second clutch and the third clutch are arranged between the first motor and the second motor.

According to the hybrid power output system as above, when the engine is driving the wheels alone, the third clutch is disengaged, that is, the second motor is disconnected with the wheel driving shaft. Therefore, all the power output from the engine can be transmitted to the wheels, it is avoided that a part of the power is wasted on the idle of the second motor, thus the power efficiency is increased.

Preferably, the engine, the first clutch, the first motor, the second clutch, the third clutch and the second motor may be connected or arranged in sequence and coaxially.

In this arrangement, the output shaft of the engine is arranged coaxially with the rotary shafts of the first motor and the second motor, and they are mechanically connected via the first clutch, the second clutch and the third clutch. Thus, the driving units of the power output system may be compactly arranged on the vehicle, and be easier to be integrated or installed. Furthermore, since all the driving units are coaxial, no special means is required for transmissions of mechanical energy (such as gear systems among the different rotating shafts), and the loss of energy during transmission is minimized, and therefore the efficiency of the power output system is increased.

Furthermore, the first motor and the second motor each can be used to propel the vehicle alone, and both can be used to propel the vehicle together. As for various driving conditions of the vehicle, the first motor and the second motor can be operated at the highest efficiency, such that the hybrid output system can realize multiple driving modes, so as to optimize the power transmission efficiency and reduce the fuel consumption.

In the present invention, it may be designed that the first motor operates more efficiently than that of the second motor at a high rotary speed; while the second motor operates more efficiently than that of the first motor at a low rotary speed. Thus, the first motor and the second motor can meet the requirements of the wheels for torque and rotary speed by cooperating at the optimum efficiency. Moreover, the design difficulties (the difficulties for designing a motor which has a high efficiency at full speeds including the low-speed, mid-speed and high-speed) for both motors can be reduced. For example, when the vehicle is started from the rest or is traveling at a low speed, the higher efficient second motor may be operated to drive the wheels alone; when the speed is up to a specified value at which the first motor can work at a higher efficiency, the first motor may be operated to drive the wheels alone; and when the speed is moderate, the first and the second motors may be cooperated at a higher efficiency for driving the wheels together.

Thus, during the purely electrical operation, the power output system will also have an excellent power output capacity to meet the requirements for propelling the vehicle, even though the internal combustion engine is not started. Thus, the fuel consumption and pollutant emission is minimized.

Furthermore, it may be designed that, the power of the second motor is greater than that of the first motor. When the second motor is operated for propelling the vehicle from the rest, a higher torque is required, therefore the power of the second motor is designed to be greater than that of the first motor.

Furthermore, the second motor can be connected to the wheel driving shaft via a constant-mesh fixed ratio reduction unit. The fixed ratio reduction unit may be a constant-mesh gear-drive reduction unit, a constant-mesh chain-drive reduction unit or a constant-mesh friction-drive reduction unit. In such an arrangement, the power to the wheel driving shaft will not be interrupted due to change of shifts, and therefore the comfort of the vehicle can be improved. Moreover, since the operation and control for changing shifts is not required, the power output system is easier to be operated and controlled. Moreover, since it is not needed to leave a large space on the vehicle to install the large sized transmission, the space arrangement of the power output system can be optimized, and the cost also can be reduced.

The other features and advantages of the present invention will be further described below with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a hybrid power output system of the present invention.

FIG. 2 to FIG. 13 are views illustrating the various power output modes of the present hybrid power output system.

FIG. 14 and FIG. 15 are views illustrating various energy recover modes of the present hybrid power output system.

FIG. 16 is a view illustrating the state in which the present hybrid power output system is used for charging the battery when the vehicle is parking.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

Referring to FIG. 1, it is a schematic view illustrating the hybrid power output system according to the present invention. The system is used to output the power to the wheel driving shaft and the wheels so as to propel the vehicle. This hybrid power output system comprises an engine 1, a first motor 2, a second motor 3, a battery 6, a first clutch 4, a second clutch 5 and a third clutch 11, wherein the first motor 2 and the second motor 3 are connected electrically with the battery 6; the engine 1 is connected to the first motor 2 via the first clutch 4; the first motor 2 is connected to a wheel driving shaft 8 via the second clutch 5; the second motor 3 is connected to the wheel driving shaft 8 via the third clutch 11; and the second clutch 5 and the third clutch 11 are arranged between the first motor 2 and the second motor 3.

As shown in FIG. 1, the engine 1, the first clutch 4, the first motor 2, the second clutch 5, the third clutch 11 and the second motor 3 may be connected in sequence and coaxially, that is, the rotating shafts of the first motor 2 and the second motor 3 are arranged coaxially with the output shaft of the engine 1. The output shaft of the engine 1 is connected with the rotating shaft of the first motor 2 via the first clutch 4, the rotating shaft of the first motor 2 is connected with the rotating shaft of the second motor 3 via the second clutch 5 and the third clutch 11, and the wheel driving shaft 8 may be arranged between the second clutch 5 and the third clutch 11.

The first motor 2 and the second motor 3 may be AC motors such as the permanent magnet synchronous motor. These two motors each can be functional as a generator for generating electricity or an electric motor for driving. The battery 6 is a controllable energy storage unit, which may be, but not limited to, a lithium battery, an iron battery, a nickel-hydrogen battery, a nickel-chrome battery and so on. The first motor 2 and the second motor 3 may be respectively connected electrically with the battery 6 through a first power converter 9 and a second power converter 10. The first power converter 9 and the second power converter 10 may have the bi-directional power conversion, thus, through the conversion of power switching unit, the motors may convert the electrical energy stored in the battery 6 into the power output, and may also convert the power into the electrical energy for storing in the battery 6.

As shown in FIG. 1, the first motor 2 is connected with the engine 1, the wheel driving shaft 8 and the battery, such that the energy can be converted or transferred among the output shaft of the engine 1, the battery 6 and the wheel driving shaft 8. When the engine 1 is required to start, the first motor 2 is coupled with the output shaft of the engine 1 via the engaged first clutch 4, and receives electrical energy from the battery 6 to produce adequate torque for starting the engine 1. When the battery 6 is required to be recharged, the first motor 2 may also generate electrical energy to charge the battery 6 under the driving of the engine 1.

It may be designed that the second motor 3 operates more efficiently than that of the first motor 2 at a low rotary speed, with the power higher than that of the first motor 2, and is mainly operated for driving the wheels alone in a purely electrical manner when the vehicle is started from the rest or is traveling at a low speed. When the second motor 3 fails to furnish sufficient torque required by the wheels, the first motor 2 can be operated to drive the wheels together with the second motor 3 in the purely electrical manner. Also, it may be designed that the first motor 2 operates more efficiently than that of the second motor 3 at a high rotary speed, such that it can be used to start the engine 1 alone, and also can be used to drive the wheels alone in the purely electrical manner.

The higher efficiency range of the engine 1 is usually at the relatively high rotary speed. Thus, during the vehicle is accelerating from the rest to the relatively high speed, the motor is used for electrically propelling the vehicle, while the engine is not required for propelling. Therefore, the engine can be avoided to operate in a low efficiency range, which may cause high consumption of fuel and high emission of pollutants.

When the first motor 2 together with the second motor 3 are used for electrically propelling the vehicle, the second clutch 5 shall be switched to be engaged. The first motor 2 and the second motor 3 can be operated to be at a substantially same speed, and then the second clutch 5 is operated to engage. During the low speed condition of the vehicle, the first motor 2 and the second motor 3 are able to provide adequate torque to meet the requirement for heavy load of the vehicle, and the engine is not required for such a heavy load, and therefore the engine is avoided to work in a low efficiency range.

The engine 1 can work using the fuel such as gasoline and diesel, and output the power through the wheel driving shaft 8.

As shown in FIG. 1, the second motor 3 is preferably connected to the wheel driving shaft 8 via a fixed ratio reduction unit 7. Preferably, the fixed ratio reduction unit 7 is located between the second clutch 5 and the third clutch 11, so as to meet the requirement for arrangement of the vehicle. The fixed ratio reduction unit may be but not limited to a constant-mesh gear-drive reduction unit, a constant-mesh chain-drive reduction unit or a constant-mesh friction-drive reduction unit, so long as the speed can be reduced and the reduction ratio is constant.

As shown in FIG. 1, the present invention may be operated in the modes of purely electrical energy operation, purely engine operation, series-hybrid operation, parallel-hybrid operation and series-parallel operation.

With reference to FIG. 2 to FIG. 13, the different operation modes of the present hybrid power output system will be described as below.

FIG. 2, FIG. 3 and FIG. 4 are the views illustrating that the driving shaft 8 is only driven based on the power output of the motor(s), which is the so-called purely electrical energy operation mode of the hybrid power system, that is, the engine 1 is stopped, only the motor is used for propelling the vehicle.

During this mode, the engine 1 is stopped, the first clutch 4 is disengaged, and the power may be flowed in three different manners, as shown in FIG. 2, FIG. 3 and FIG. 4 respectively.

As shown in FIG. 2, it shows the operation mode in which both the first motor 2 and the second motor 3 are operated for propelling the driving shaft 8, at this time the second clutch 5 and the third clutch 11 is engaged. Thus, the rotating shafts of both motors may simultaneously transfer the torque to the driving shaft 8; the first motor 2 and the second motor 3 work under the electrical energy operation mode, the battery 6 provides the electrical energy to both motors. As indicated by the arrow in FIG. 2, the battery 6 provides the electrical energy to the first motor 2 and the second motor 3, and then the latter may output the torque to the driving shaft 8.

As shown in FIG. 3, it shows the operation mode in which only the second motor 3 is operated to propelling the vehicle, at this time, the second clutch 5 is disengaged, the third clutch 11 is engaged, the first motor 2 is stopped, the second motor 3 is operated under the electrical energy operation mode, the battery 6 provides the electrical energy for the second motor 3. As indicated by the arrow in FIG. 3, the battery 6 provides the electrical energy to the second motor 3, and the second motor 3 output the torque to the driving shaft 8.

FIG. 4 shows the operation mode in which only the first motor 2 is operated to propelling the vehicle, at this time, the second clutch 5 is engaged, the third clutch 11 is disengaged, the second motor 3 is stopped, the first motor 2 is operated under the electrical energy operation mode, the battery 6 provides the electrical energy for the first motor 2. As indicated by the arrow in FIG. 4, the battery 6 provides the electrical energy to the first motor 2, and the first motor 2 output the torque to the driving shaft 8. Because the second motor 3 is disconnected from the wheel driving shaft 8, the second motor 3 will not be idle and, as a result, it will be avoided to waste energy.

FIG. 5 and FIG. 6 show operation mode in which the engine 1 is operated, but the driving shaft 8 is only driven by the second motor 3, which is usually called as the series-hybrid operation mode.

During this mode, the second clutch 5 is disengaged, the third clutch 11 is engaged, thus the mechanical connection between the first motor 2 (and the engine 1) and the driving shaft 8 is disconnected; the first clutch 4 is engaged, connecting the output shaft of the engine 1 with the rotating shaft of the first motor 2; the power from the engine 1 is transferred to the first motor 2, the first motor 2 is operated in the generating mode, the second motor 3 is operated in the electrical energy operation mode. As indicated by the arrow in FIG. 5, when the battery 6 is required to be charged, partial electrical energy from the first motor 2 is used to charge the battery 6, while the remain is transferred to the second motor 3, the second motor 3 provides the torque to the driving shaft 8. As indicated by the arrow in FIG. 6, the electrical energy from the first motor 2 is transferred to the second motor 3, the battery 6 also gives the electrical energy to the second motor 3, and the second motor 3 transfers the torque to the driving shaft 8.

FIG. 7, FIG. 8 and FIG. 9 show the operation mode in which the engine 1 is used to propel the driving shaft 8, and at the same time, the electrical energy from the battery 6 is also consumed to produce the drive force to the driving shaft 8, which is usually called as the parallel-hybrid operation mode.

During this mode, both the first clutch 4 and the second clutch 5 are engaged, the third clutch 11 is engaged or disengaged, thus enable the output shaft of the engine 1 mechanically connected with the rotating shaft of the first motor 2, enable the first motor 2 (or the second motor 3) mechanically connected with the driving shaft 8. According to the different operation modes of the first and the second motors, the power can be flowed in different manners.

FIG. 7 shows the first power flow manner, wherein the first clutch 4, the second clutch 5 and the third clutch 11 are all engaged, and the first motor 2 and the second motor 3 both are working under the battery energy; the battery 6 provides the required electrical energy for the two motors. As indicated by the arrow in FIG. 7, the engine 1 functions and transfers the torque to the driving shaft 8 through the rotating shaft of the first motor 2. According to the required drive torque, the first motor 2 and the second motor 3 may convert the electrical energy from the battery 6, and output the torque to the driving shaft 8.

FIG. 8 shows the second power flow manner, wherein the first clutch 4, the second clutch 5 and the third clutch 11 are all engaged. The first motor 2 is idle, the second motor 3 operates under the battery energy; the battery 6 may provide the electrical energy as required. As indicated by the arrow in FIG. 8, the engine 1 functions to transmit the torque to the driving shaft 8 through the rotating shaft of the first motor 2. According to the drive torque as required, the second motor 3 may receive the electrical energy from the battery 6 to provide torque to the driving shaft 8.

FIG. 9 shows the third power flow manner, wherein the third clutch 11 is disengaged, while the first clutch 4 and the second clutch 5 are engaged. The second motor 3 is disconnected from the wheel driving shaft 8, and the first motor 2 operates under the battery energy; the battery 6 may provide the electrical energy as required. As indicated by the arrow in FIG. 9, the engine 1 functions to transmit the torque to the driving shaft 8 through the rotating shaft of the first motor 2. According to the drive torque as required, the first motor 2 may receive the electrical energy from the battery 6 to provide torque to the driving shaft 8.

FIG. 10 and FIG. 11 show the operation mode in which the engine 1 output the power, the first motor 2 is generating the electricity, and the second motor 3 operates under the electrically driving mode, which is usually called as the series-parallel operation mode.

In this mode, partial power of the engine 1 is directly transferred to the driving shaft, the remain is used for driving the first motor 2 to work under the electricity-generating mode, and the second motor 3 is operating in the electrical energy mode. As indicated by the arrow in FIG. 10, partial electrical energy generated by the first motor 2 is directly transferred to the second motor 3, enable the second motor 3 electrically drive the wheels, and the remain is charging the battery 6. As indicated by the arrow in FIG. 11, the electrical energy generated by the first motor 2 is directly sent to the second motor 3, and the battery 6 also provides the electrical energy to the second motor 3, such that the second motor 3 electrically drive the wheels.

FIG. 12 and FIG. 13 show the operation mode in which the driving shaft 8 is only propelled by the engine 1, which is usually called as the purely engine operation mode, and at this time, both the first clutch 4 and the second clutch 5 are engaged, and the third clutch 11 is disengaged. Because the second motor 3 is disconnected from the wheel driving shaft 8, the second motor 3 will not be idle and, as a result, it will be avoided to waste energy.

FIG. 12 shows the operation mode in which the first motor 2 is idle and the second motor 3 is stopped, and the rotating shaft of the first motor 2 is used to transfer the torque from the engine 1 to the driving shaft 8. As indicated by the arrow in FIG. 12, the engine 1 functions and transfers the torque to the driving shaft 8 through the rotating shaft of the first motor 2.

FIG. 13 shows the operation mode in which the first motor 2 transfers partial power from engine 1 into electrical energy for charging the battery 6, and at the same time its rotating shaft transfers the remaining power from the engine 1 to the driving shaft 8. As indicated by the arrow in FIG. 13, the engine 1 starts, and a part of the power thereof is transferred to the driving shaft 8 through the rotating shaft of the first motor 2, and the other part of the power is converted into electrical energy by the first motor 2 and stored in the battery 6.

FIG. 14 and FIG. 15 are views illustrating the energy recover operation mode of the present invention.

When the vehicle is decelerating or braking, the present invention can be used to recover energy. In this mode, the motor works in electricity generating mode, and transfers the power feedback from the driving shaft 8 into the electrical energy for charging the battery 6. According to the engaged/disengaged status of the second clutch 5, the energy may be recovered in different manners.

FIG. 14 shows the energy recover mode in which both the second clutch 5 and the third clutch 11 are engaged. In this mode, the rotating shafts of both the first motor 2 and the second motor 3 are connected with the driving shaft 8, and the first motor 2 and the second motor 3 are working under the electricity generating mode, for storing the recovered electricity to the battery 6. As indicated by the arrow in FIG. 14, the driving shaft 8 feedbacks the mechanical energy from the wheels to the first motor 2 and the second motor 3, and the latter is operated to convert the mechanical energy into electrical energy for charging the battery 6 to realize the energy recover purpose.

FIG. 15 shows the energy recover mode in which the second clutch 5 is disengaged and the third clutch 11 is engaged. In this mode, the rotating shaft of the second motor 3 is connected with the driving shaft 8, the second motor 3 is working under the electricity generating mode, for storing the recovered electrical energy to the battery 6. As indicated by the arrow in FIG. 15, the driving shaft feedbacks the mechanical energy from the wheels to the second motor 3, and the second motor 3 converts the mechanical energy into the electrical energy for charging the battery 6 to realize the energy recover purpose.

FIG. 16 is a view illustrating the operation mode in which the battery is charging while the vehicle is parking.

In this mode, the second clutch 5 is disengaged and the second motor 3 is stopped, the engine 1 drives the first motor 2 to work under the electricity generation mode, and stores the electrical energy into the battery 6. As indicated by the arrow in FIG. 16, the engine 1 starts to drive the first motor 2, and the first motor 2 is driven to convert the power from the engine 1 into the electrical energy for charging the battery 6.

Although the invention has been described with respect to specific embodiments, the details are not to be construed as limitations, for it will become apparent that various embodiments, changes and modifications may be resorted to without departing from the spirit and scope thereof, and it is understood that such equivalent embodiments are intended to be included within the scope of this invention. 

1-9. (canceled)
 10. A hybrid power output system for a vehicle, comprising: a first motor; a second motor; a battery electrically connected to the first motor and the second motor; an engine; a wheel driving shaft configured to provide rotational power to at least one wheel; a first clutch operatively coupled between the engine and the first motor; a second clutch operatively coupled between the first motor and the wheel driving shaft; a third clutch operatively coupled between the second motor and the wheel driving shaft; and wherein the second clutch and the third clutch are operatively coupled between the first motor and the second motor.
 11. The system of claim 10, wherein the engine, the first clutch, the first motor, the second clutch, the second motor, and the third clutch are connected in sequence in a coaxial arrangement.
 12. The system of claim 10, wherein the first motor is a mid-speed or a high-speed motor, and the second motor is a mid-speed or a low-speed motor.
 13. The system of claim 10, wherein the first motor has sufficient output power so as to propel the vehicle without engaging the second motor, and the second motor has sufficient output power so as to propel the vehicle without engaging the first motor.
 14. The system of claim 10, wherein the first motor and the second motor are configured to propel the vehicle simultaneously.
 15. The system of claim 10, wherein an output power of the second motor is greater than an output power of the first motor.
 16. The system of claim 10, wherein the first motor and the second motor are electrically connected to the battery, respectively, through a power converter.
 17. The system of claim 10, wherein the second motor is operatively coupled to the third clutch, the third clutch is operatively coupled to a constant-mesh fixed ratio reduction unit, and the constant-mesh fixed ratio reduction unit is operatively coupled to the wheel driving shaft.
 18. The system of claim 18, wherein the fixed ratio reduction unit is a constant-mesh gear-drive reduction unit, a constant-mesh chain-drive reduction unit, or a constant-mesh friction-drive reduction unit.
 19. A hybrid power output system for a vehicle, comprising: a first motor; a second motor; a battery electrically connected to the first motor and the second motor; an engine; a constant-mesh fixed ratio reduction unit configured to receive rotational input power and transmit rotational power to a wheel driving shaft; a first clutch operatively coupled between the engine and the first motor; a second clutch operatively coupled between the first motor and the wheel driving shaft; a third clutch operatively coupled between the second motor and the constant-mesh fixed ratio reduction unit; and wherein the second clutch and the third clutch are operatively coupled between the first motor and the second motor.
 20. The system of claim 19, wherein the fixed ratio reduction unit is a constant-mesh gear-drive reduction unit, a constant-mesh chain-drive reduction unit, or a constant-mesh friction-drive reduction unit.
 21. The system of claim 19, wherein the engine, the first clutch, the first motor, the second clutch, the second motor, and the third clutch are connected in sequence in a coaxial arrangement.
 22. The system of claim 19, wherein the first motor is a mid-speed or a high-speed motor, and the second motor is a mid-speed or a low-speed motor.
 23. The system of claim 19, wherein the first motor has sufficient output power so as to propel the vehicle without engaging the second motor, and the second motor has sufficient output power so as to propel the vehicle without engaging the first motor.
 24. The system of claim 19, wherein the first motor and the second motor are configured to propel the vehicle simultaneously.
 25. The system of claim 19, wherein an output power of the second motor is greater than an output power of the first motor.
 26. The system of claim 19, wherein the first motor and the second motor are electrically connected to the battery, respectively, through a power converter.
 27. A hybrid power output system for a vehicle, comprising: a first motor; a second motor; a battery electrically connected to the first motor and the second motor; an engine; a constant-mesh fixed ratio reduction unit configured to receive rotational input power and transmit rotational power to a wheel driving shaft; a first clutch operatively coupled between the engine and the first motor; a second clutch operatively coupled between the first motor and the wheel driving shaft; a third clutch operatively coupled between the second motor and the constant-mesh fixed ratio reduction unit; and wherein the second clutch and the third clutch are operatively coupled between the first motor and the second motor, and wherein the engine, the first clutch, the first motor, the second clutch, the second motor, and the third clutch are connected in sequence in a coaxial arrangement.
 28. The system of claim 27, wherein the first motor is a mid-speed or a high-speed motor, and the second motor is a mid-speed or a low-speed motor.
 29. The system of claim 27, wherein the first motor has sufficient output power so as to propel the vehicle without engaging the second motor, and the second motor has sufficient output power so as to propel the vehicle without engaging the first motor. 